Vitreous coating



Patented Apr. 5, 1949 UNITED STATES FATENT @EFICE VITBEOUS COATING of iennsylvania No Drawing.

Application March 4, 1942,

Serial No. 433,326

1 Claim.

This application is a continuation in part of applications filed by meJuly 28, 1938, Serial No. 221,793, now abandoned, and February 17, 1940,Serial No. 319,517, now abandoned.

In the production of enamel ware, that is to say metal (typically sheetsteel) coated with vitreous enamel fused upon its surface, animperfection is likely to occur that is known as hairlining or tearing.In the finished article dark lines are apt to appear, lines Where therehas been a break in the continuity of the coating material, with theeffect that underlying material shows through. This is of importance,particularly in White or light-colored enamel ware.

I have made considerable investigation and have perceived the causes ofthe defect, and my invention lies in the discovery of preventive means.

In typical procedure the article of sheet steel is coated first with aground coat and fired, and then the ground-coated article is coated witha slip or sprayed-on coating of fluid enamel. This iluid enamel is adispersion in liquid (ordinarily Water) of very fine ground vitreousmaterial, together with (I speak of White enamel) an opacifyingmaterial, typically tin oxide. Additionally, about 7 -72; of clay isused as a suspending agent in preparing the slip.

Procedure consists first in coating the article with the slip, thendrying, leaving the coating in caked condition upon the surface, andfinally firing. In the firing the Vitreous material fuses, and in fusedcondition becomes permanently adherent to the surface.

In the drying of the spr ad slip and in the handling and in the initialstages of firing, some warping of the steel body is almost unavoidable,and in consequence of Warping the caked coating cracks, and the cracksso developed persist in the flaw that is known as hair-lining.

Fineness of grinding of the frit is desirable, in that maturingtemperature is lowered, superior surface gloss is attained, and, in thecase of opaque enamel, opacity is increased; it has, however, been foundthat fineness of grinding tends to make more pronounced the defect oftearing or hair-lining, and, accordingly, it has heretofore been foundpractically necessary to limit fineness of grinding to the point atwhich, if cc. of the milled enamel (=approximately -95 grams) be washedthrough a screen of 200 meshes to the inch, there will remain on thescreen a residue of 8-12 grams of the enamel.

It also is desirable that the slip spread upon the surface of thearticle be, within the limits in contemplation, of maximum thickness;but it been found that, as thickness increases, the tendency to tearingincreases; so that, in consequence, slip thickness is in practicelimited to a Weight of ground frit of about 35 grams to the square foot.

It in the provision of cover-coats in the production of enameled steelarticles that the dimculty of hair-lining arises. The frit for suchcoats should be so compounded as to mature at a temperature of 1660 F.or less. An important ingredient of the frits for this purpose is borax.Borax being present, lowers the temperature at which the frit matures toform the cover coat. In the course of grinding of the frit components toproduce a mix of the desired degree of fineness, the borax tends toleach out. The finer the grinding and the higher the temperature of thematerial When subject to the grinding operation, the greater is theleaching effect upon the borax. The borax so leaching into the millliquor has the effect that the irit when fired tends to hairline. Thistendency is aggravated by increase in the thickness of the appliedcoating, and, of course, by thermal expansion and contraction of thearticle after coating and before firing. If an enamel is so compoundedthat the leaching of the borax is reduced beyond a certain value,hair-lining does not occur.

It is known that the solubility of borax is infiuenced by the presenceof silica and alumina (alumina is introduced into the frit compositionin association with silica in the form of feldspar) and it these twoconstituents, silica and feldspar, be present in the frit in combinedquantity not substantially exceeding 50% of the Whole, and if borax bepresent in quantity (as the art knows) sufficient to bring the maturingtemperature to 1608" F. or less, the enamel may be made subject to myinvention.

tion and character indicated a particular sub-.

stance be added, the frit may be ground more finely and the slip may beapplied in .a coating of greater thickness, and yet the defect ofhairlinin will not ensue. The substance added is a material ofrelatively low melting point, typically sodium nitrite (a compound thatmelts at about 520 F.). The sodium nitrite is added in amount varyingfrom 0.125% to 1% by weight of the frit. It is intimately mingled Withthe finely divided glass, and is preferably, though not necessarily,combined with the glass before the glass is ground with water to thecondition of a slip. It may, however, be added, dissolved in water, tothe already milled slip. It is itself water-soluble and in the slip,however it be added, it comes to dissolved state. When the article hasbeen coated with material that carries such sodium-nitrite content andis at the beginning of the firing operation, the early fusion of thesodium nitrite efiects such an early adhesion of the coating to theunderlying surface that the cleavages productive of hair lines either donot occur or, having occurred, do not develop, and the coating is foundto be continuous, and the product free of the cracks underconsideration.

In the firing of coats of enamel that are compounded as usual and lackthe modification in which my present invention is found, the coating inthe early stages of firing becomes crazed with a network of finefissures. These tend to intensity the fissures that endure ashair-lines. In the firing of a coating of my invention such craZing inthe initial stages of firing does not occur.

I have found that, if I make such an addition of sodium nitrite, thegrinding of the frit may be carried farther, and with the recognizedadvantages of so doing, and yet without sufiering the countervailingliability of the enamel to tearing or hair-lining. I have found that,with the aid of the sodium nitrite addition, a frit ground to a finenessmore minute than that defined above will, when worked to a slip andapplied, still be serviceable and free of the defect of hair lining; Ihave found that the frit may be ground to such degree of fineness thatit will pass substantially in toto through a ZOO-mesh screen, and yettearing or hair-lining will not develop. I have further found that withthe sodium nitrite addition the slip may be applied in a coating asthick as 40-70 grams of frit to the square foot, and yet the firedarticle will be free of hair-lining.

Incidentally, in the firing of ground coats upon articles of steel, thepresence of nitrite of sodium in the ground coat (producing fusion atlower temperature) tends to give a better union of the coating to themetal, that is to say, the bonding eifect is improved, in that itpermits of wider deviations from normal practice without detriment.

When an article of steel, upon whose surface lies a hardened coating ofusual composition, is introduced into the furnace and fired, as thetemperature mounts, air, gaining access through the porous coating,produces iron oxide at the metal-coating interface. In the practice ofmy invention the fusion at relatively low temperatures of the sodiumnitrite content of the coating has eifect in sealing the steel surfacefrom access of air, and so in diminishing the extent of such oxidation.

The following are exemplary formulae for frit upon which my inventionmay with success be practiced:

Clear enamel Per cent Silica 20.15 Titanium oxide 3.70 lSoda ash 6.10Cryolite 5.55 Dehydrated borax 16.00 Fluorspar 4.63 Zinc oxide 3.70Nitrate of soda 4.62 Feldspar 30.43 Whiting 1.42 Sodium silico-fiuoride3.70

Total 100.00

Opacified enamel Per cent Silica 18.59 Titanium oxide 1.86 Soda ash 4.18Cryolite 7.06 Dehydrated borax 16.02 Fluorspar 11.70 Nitrate of soda6.60 Feldspar 23.59 Barium carbonate 2.41 Antimony trioxide 7.99

Total 100.00

In each of these cases, if to the frit nitrite of sodium be added in thequantity specified, hairlining, to which the applied enamel wouldotherwise be liable, will not occur.

The modifying material must meet certain conditions: it must be of lowmelting point (not exceeding and preferably lower than 1000 F.); it mustbe chemically inert, to this extent, that at its own melting point itwill not react with the glass, nor with the metal of the article beingcoated; and it must have neither disfiguring nor discoloring effect. Tomeet these requisites a wide range of particular materials is available.So far as I have investigated, no elemental substance will serve; butthere are many compounds that will serve. For practical purposesmetallic salts are most readily available, and I specify particularly,in addition to sodium nitrite, potassium sulphocyam'de, whose meltingpoint is about 343 F., other alkali-metal sulphocyanides, andalkali-metal salts of low melting point generally, and stannouschloride, with a melting point of about 447 F. In all of these cases thequantity will be within the range specified above for sodium nitrite(0.125400%).

The invention, developed in the enamel-ware industry, is of generalapplicability in the formation of vitreous coatings upon bodies ofvarious substances, both metallic (cast iron, for example) andnon-metallic (pottery, for example).

I believe that I am the first to discover the value of sodium nitriteand the equivalent substances specified, in the manner and under thecircumstances described and with the eifect of preventing tearing orhair-lining.

I claim as my invention:

A slip for the enameling of a steel article, formed from a frit thatincludes feldspar and silica in combined quantity not substantiallyexceeding 50% and that includes also a borax, and of a maturingtemperature not exceeding 1600 F., fortified against tearing by theinclusion of a-salt of an alkali metal having a melting point notexceeding 1000 F., the salt of the alkali metal employed being asulpho-cyanide and the quantity being 0.125400%,

JACOB E. ROSENBERG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,010,776 Frost Aug. 6, 19352,250,456 Bahnsen et a1 July 29, 1941 2,250,457 Bahnsen et a1 July 29,1941 FOREIGN PATENTS Number Country Date 297,725 Great Britain 1929 5643,805 Germany 1937 OTHER REFERENCES Andrews: Enamels, pages 260, 378,379 (1935).

Journal American Ceramic Society: The

10 Tearing of Vitreous Enamels, by Rosenberg 8:

Langerman, v01. 23 (3) 83-86, March 1940.

